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Development of in-situ photoluminescence characterization tools for the study of semiconductors for photovoltaics application

Abstract : During the last few decades, the conversion efficiency of photovoltaic solar cell has been significantly improved, almost reaching the Shockley-Queisser theoretical limit. At this point, a profound understanding of material properties and its evolution during the solar cell fabrication processes become increasingly crucial to further improve the cell conversion efficiency. For this reason, my doctoral studies have been focused on the development of in-situ characterization tools, which allows the studies of material properties in real time during the processes. The tools were developed based on photoluminescence techniques, in which the sample (semiconductor materials) is optically excited and simultaneously emits photons with energy approximately equal to the band gap of materials. In this thesis, three in-situ characterization tools will be presented.In-situ SSPL, based on steady-state photoluminescence technique, is developed to study the properties of semiconductor materials during the processes by directly measuring the steady-state PL intensity. After the upgrade of optical acquisition system, the tool has been used extensively to study the evolution of surface properties of crystalline silicon wafer, passivated by aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) and hydrogenated amorphous silicon (a-Si:H) grown by plasma-enhanced chemical vapor deposition (PECVD), under Ar/H2 plasma exposure at different conditions. From these experiments, the behavioral differences between Al2O3-passivated sample and a-Si:H-passivated sample was observed and discussed. In addition, thanks to the plasma exposure experiment through a magnesium fluoride optical window (MgF2), the root cause of plasma-induced degradation of surface passivation was pin-pointed. Last but not least, the relationship between the dynamic of plasma-induced degradation and the plasma parameters (e.g. applied RF power, chamber pressure, and temperature) was also studied.To go one step further, in-situ MPL, based on modulated photoluminescence technique, is built to quantitatively study the properties of semiconductor materials. This characterization tool employs an intensity-modulated laser to excite the sample, so the minority carrier lifetime can be measured. After the conceptualization and fabrication of a new optical acquisition system, the system calibration and the optimization of MPL parameters were conducted. Furthermore, a characterization method was also developed, so the in-situ MPL is able to measure the minority carrier lifetime at a defined minority carrier density (e.g. 1015 cm-3 for non-concentrated single junction solar cell). After a lot of work, the tool is now fully functional and has been used to measure the minority carrier lifetime of crystalline silicon wafer during the de position of a-Si:H passivation layer, the thermal treatment, and the deposition of hydrogenated amorphous silicon nitride (a-SiNx:H) anti-reflection coating. The experimental results show that the temperature at which the processes were conducted plays a major role in activation and modification of surface passivation properties provided by Al2O3.Finally, as the tendency toward tandem solar cell has been continuously growing, another in-situ characterization tool, known as in-situ PLt (in-situ photoluminescence for tandem solar cell), was built. This characterization tool results from a combination of steady-state photoluminescence and modulated photoluminescence technique and was designed to study in real time the properties of both sub-cells independently and simultaneously. The in-situ PLt can be a potential characterization tool for the research toward high efficiency tandem solar cell.
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Mengkoing Sreng. Development of in-situ photoluminescence characterization tools for the study of semiconductors for photovoltaics application. Materials Science [cond-mat.mtrl-sci]. Institut Polytechnique de Paris, 2019. English. ⟨NNT : 2019IPPAX003⟩. ⟨tel-02496035⟩

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